The invention relates to a process for preparing polysilanes from monosilanes in the presence of Lewis-acid boron compounds.
There are a number of synthetic methods for preparing polysilanes:
The most widely used is the Wurtz-type coupling of chlorosilanes with elemental alkali metals in high-boiling solvents. This was described for the first time in F. S. Kipping (1921) J Chem Soc 119:830. However, carrying out the reaction is extremely complicated because of the harsh reaction conditions and the high reactivity or sensitivity of the starting materials. Furthermore, the product distribution is not homogeneous and modification of the polysilane can only be effected by use of other monomers. However, only monomers in which the functional groups are retained under strongly reducing reaction conditions are suitable for this purpose, which greatly restricts the choice of suitable chlorosilanes.
Furthermore, hydrosilanes can be coupled in the presence of transition metal catalysts with elimination of hydrogen to form polysilanes. This polymerization method has been known only since the end of the 1990s from J. F. Harrod, C. Aitken, E. Samuel (1985) J Organomet Chem 279:C11. The air- and water-sensitivity of the transition metal catalysts and the fact that virtually only primary silanes are suitable for this type of polymerization significantly restricts the possibilities for this type of polysilane synthesis. In addition, the strong hydrogen evolution in the polymerization in bulk and the resulting foaming of the polymer with a simultaneous decrease in viscosity can make carrying out the reaction difficult.
Furthermore, the catalyst residues cannot be completely separated off from the polysilane.
Polysilanes can also be prepared from masked disilenes by means of an extremely complicated monomer synthesis.
This type of polymerization was described for the first time in K. Sakamoto, K. Obata, H. Hirata, M. Nakajima, H. Sakurai (1989) J Am Chem Soc 111:7641. However, in this case the monomer synthesis limits the choice of substituents on the silicon to a similarly great degree as in the Wurtz-type coupling. Thus, neither an advantageous monomer synthesis nor a flexible silane polymerization is possible by means of this method.
DE 102006034061 A1 describes a technologically highly demanding plasma process for preparing polysilane from SiCl4 and H2. However, this process is costly and energy-intensive and requires both complex purification of the product and polymer-analogous reactions for converting the perchlorinated polysilane into the actual polysilane. Production of carbon-containing polysilanes is quite impossible.
A further possibility for the polysilane synthesis is the ring-opening polymerization of silacycles, but this method again has the disadvantage that the monomer synthesis is relatively complicated and a free choice of the substituents is once again not possible. This was firstly described in M. Cypryrk, Y. Gupta, K. Matyjaszewski (1991) J Am Chem Soc 113:1046. In addition, there is the possibility of reacting dichlorosilanes either electrochemically as described by M. Ishifune, S. Kashimura, Y. Kogai, Y. Fukuhara, T. Kato, H. B. Bu, N. Yamashita, Y. Murai, H. Murase, R. Nishida (2000) J Organomet Chem 611:26, or stepwise with dilithiated silane species to form polysilane as described by J. P. Wesson, T. C. Williams (1981) J Polym Sci, Part A: Polym Chem 19:65. The electrochemical procedure is energy-intensive, while the stepwise formation of the polysilane is very complicated and suitable virtually only for the laboratory scale. Both methods require, like the plasma synthesis, a polymer-analogous reaction of the perhalogenated polysilane.
Problems common to these known synthetic methods are the purification of the polymer, low molar masses and an excessively heterogeneous product distribution. In particular, the purity of the polymer in respect of metal contamination caused by the catalyst makes them suitable to only a limited extent for efficient use in industrial/electronic components.
In addition, free variation of the substituents on the polymer backbone and thus controlled setting of the carbon content is possible to only a limited extent in the known synthetic roots. This is, according to D. R. Miller, J. Michl (1989) Chem Rev 89:1359, attributable firstly to the sometimes extremely harsh reaction conditions and secondly to the nature of the catalysts used.